The present invention relates generally to a signal selecting circuit, provided in a disk drive, which outputs a predetermined signal to a host computer and the disk drive having such a signal selecting circuit, and more particularly to a signal selecting circuit which selectively outputs a predetermined signal to a host computer compatible with a plurality of computer hardware types and a disk drive having such a signal selecting circuit.
In a conventional computer system comprising a so-called floppy disk drive (abbreviated FDD hereinafter), an interface connected to the FDD, and a host computer connected to the interface, the host computer judges if it can access the FDD by means of detecting either a ready signal (abbreviated RDY signal hereinafter) or a disk change signal (abbreviated DC signal hereinafter). Hereupon, the term "RDY signal", as used herein, means a signal representing a floppy disk is loaded into the FDD and thus information can be recorded thereon and/or reproduced therefrom. On the other hand, the term "DC signal", as used herein, means a signal representing the floppy disk is ejected from the FDD. Whether the host computer detects the RDY signal or the DC signal is predetermined by a hardware type of the host computer. For example, an NEC PC9801 detects the RDY signal, and an IBM PC/AT detects the DC signal. Jumper lines or short bars (strap pins) are provided in the FDD, and consequently the FDD outputs the signal corresponding to the hardware type of the host computer thereto via the jumper line (or the short bar) and the interface.
A storage capacity of a floppy disk used in the FDD is also predetermined by the hardware used in the host computer. For example, a high density type (2HD) disk having a storage capacity of 1.6 MB and/or a normal density type (2DD) disk having a storage capacity of 1.0 MB, before the disk is formatted, is used for the host computer if it is the NEC PC9801. On the other hand, a high density type (2HD) disk having a storage capacity of 2.0 MB and/or a normal density type (2DD) disk having a storage capacity of 1.0 MB, before the disk is formatted, is used for the host computer if it is the IBM PC/AT. Incidentally, if the FDD uses the 2.0 MB disk, the FDD is set in a 2.0 mode. Likewise, if the FDD uses the 1.6 MB disk, the FDD is set in a 1.6 mode, and if the FDD uses the 1.0 MB disk, the FDD is set in a 1.0 mode. In addition, for example, a term "2.0/1.0MB" means that the FDD has two modes so that it can be selectively set in the 2.0 mode or the 1.0 mode.
However, a host computer compatible with a plurality of computer hardware types, such as the NEC PC9801 and the IBM PC/AT, has been increasingly required. The FDD which can only output the RDY signal or the DC signal cannot be used for such a host computer. Additionally, a "2.0/1.6/1.0 MB" FDD which can be selectively set in one of the three modes is necessary. Although the 2.0/1.6/1.0 MB FDD has been already on sale, the FDD which has a signal selecting circuit which selectively outputs the RDY signal or the DC signal has not been developed yet. In a 2.0/1.6/1.0 MB FDD, the 2HD mode and the 2DD mode are switched by detecting the presence of a discriminating hole in the disk. Moreover, the 2.0 mode and the 1.6 mode are switched based on an external signal suppled from the host computer. However, since the FDD can only output the RDY signal, it cannot communicate with a host computer using a IBM PC/AT type. On the other hand, a computer system in which the FDD simultaneously outputs both the RDY signal and the DC signal to the host computer and the host computer selects either signal has been proposed. However, this computer system also has a disadvantage in that, since it is generally necessary to provide an extra output terminal for the FDD so as to simultaneously output both RDY signal and DC signal, the FDD cannot be compatible with the conventional interface. As a result, the computer system is made expensive and has complicated hardware.